UV Editor - Use other Units of measurements
This doesn't exist yet.
I will be submitting a pull request to the Blender3D Repo for version 2.80+
It will allow you to enter:

MM, CM, M, KM, Inches, Foot/Feet and Yards


with a screen DPI of 96 DPI
it will take input: 20cm
and it will output: 756px

Using this formula:

dpi = current monitor/screen or blenders dpi setting
scale = getScaleFromInputField() * 2.54
pixels = RoundUp(input * (dpi / scale))

getScaleFromInputField will convert MM, M, KM, Inches, Foot/Feet and Yards To Centimeters and return the correct value
Using this conversion chart:

You can contribute to my repo: https://github.com/DeanVanGreunen